Current radios function in channel environments that may contain data, clear voice, and coded or encrypted voice transmissions. For each one of these types of transmissions there is an associated attack time (i.e. the data detector takes a certain amount of time to detect) at the receiving subscriber unit. The coded voice detector may take some time to detect encrypted voice which may require more time than for clear voice to be detected.
Conventionally, the receiving subscriber unit cannot tell which type of signal will be received next. Currently, in receivers of a voice communication systems data muting typically has been accomplished by detecting a presence of a data signal and then muting the output of the receiver so that it is not heard, regardless of the following signal. As described in U.S. Pat. No. 4,430,742 and hereby incorporated by reference, the data muting is terminated whenever the last bit of the data signal has been detected (prior art #2 as seen in FIGS. 2A-C). Many different techniques exist for detecting the presence of the data signal such as detection of a word sync (as illustrated in U.S. Pat. No. 4,430,742) or in some improved methods, the detection of a bit sync (as illustrated by prior art #2 as seen in FIGS. 2A-C as another example). In addition, a timer is also used to mute the speaker for some time period (prior art #1 illustrated in FIGS. 2A-C) starting from the reception of a carrier signal and ending when data has been detected.
However, since techniques of data muting requires the reception of several bits in order to detect a data signal (attack time), a short data burst is heard by the listener with the exception of the data received in the first data packet (as covered by prior art #1 of FIGS. 2A-C). Since the peak to peak signal level of the data signal is very high for reliability reasons, the data bursts are very loud during peak periods of data traffic in such communication systems. These data bursts are then very annoying to the listener.
In the case of coded voice transmissions, there is also associated with it an attack time to decrypt the coded voice signal which results in another short data burst, which is again passed to a listener. The effect of this data burst or squelch head can be removed by muting the output of the receiver (setting a fixed speaker mute delay) for a fixed time period that is long enough to include the decryption of the coded voice signal. The fixed time period would be set after a received data packet or at the beginning of the received signal (prior art #1 of FIGS. 2A-C). This method can also mute the data bursts caused by the attack time of multiple data packets as described previously.
However, the problem with this fixed period method is that the delay has to be set at the maximum attack time for the various transmission signals. If a clear voice signal is sent instead, this fixed delay might cut off the beginning of the clear voice transmission after a data packet. Since the fixed time period must be made long enough to accommodate system delays and the longest attack time, clear voice communications having a shorter attack time and following the data signal will also be muted.
Therefore, there is a long felt need for a data muting method and apparatus that mutes the entire data signal along with unwanted data bursts without muting the following voice communications.